The present invention relates generally to touchscreens and, more particularly, to a method for minimizing the impact of surface contaminants on the operation of an acoustic touch sensor.
Touchscreens are used in conjunction with a variety of display types, including cathode ray tubes (i.e., CRTs) and liquid crystal display screens (i.e., LCD screens), as a means of inputting information into a data processing system. When placed over a display or integrated into a display, the touchscreen allows a user to select a displayed icon or element by touching the screen in a location corresponding to the desired icon or element. Touchscreens have become common place in a variety of different applications including, for example, point-of-sale systems, information kiosks, automated teller machines (i.e., ATMs), data entry systems, gaming machines, ticketing machines, etc.
A specific type of touchscreen, an acoustic touchscreen, has a touch-sensitive area on which the occurrence and location of a touch is sensed via the touch""s effect on acoustic waves propagating across the touchscreen surface. A common type of acoustic touchscreen employs Rayleigh waves (a term which, as used herein, subsumes quasi-Rayleigh waves). Illustrative disclosures relating to Rayleigh wave touchscreens include Adler, U.S. Pat. Nos. 4,642,423; 4,645,870; 4,700,176; 4,746,914; 4,791,416; and U.S. Pat. No. Re 33,151; Adler et al., U.S. Pat. Nos. 4,825,212; 4,859,996; and 4,880,665; Brenner et al., U.S. Pat. No. 4,644,100; Davis-Cannon et al., U.S. Pat. No. 5,739,479; and Kent, U.S. Pat. Nos. 5,708,461 and 5,854,450. Acoustic touchscreens employing other types of acoustic waves such as Lamb or shear waves, or combinations of different types of acoustic waves (including combinations involving Rayleigh waves) are also known, illustrative disclosures including Kent, U.S. Pat. Nos. 5,591,945 and 5,854,450; Knowles, U.S. Pat. Nos. 5,072,427; 5,162,618; 5,177,327; 5,243,148; 5,329,070; and 5,573,077; and Knowles et al., U.S. Pat. No. 5,260,521. The documents cited in this paragraph are incorporated herein by reference for all purposes.
Touchscreen applications may require that the touchscreen operate in an outdoor environment or in a relatively harsh indoor environment such as a factory or restaurant. Therefore the touchscreen may be subjected to a variety of contaminants. Unfortunately, when a surface contaminant, e.g., water, chewing gum, etc., touches the surface of an acoustic touchscreen, the system may register the contaminant as a valid touch. Unless the system recognizes that the touch is a false touch, for example by determining that the touch location remains unchanged for an extended period of time, the touchscreen may become inoperable and unable to register further touches. Even if the system recognizes that the touch is due to a contaminant, in a conventional touchscreen system typically a pair of dead zones will be created based on the coordinates of the contaminant. Consequently, regions of the touchscreen become unusable, thus compromising the performance of the touchscreen.
What is needed in the art is a method for minimizing the impact of contaminants on the performance of an acoustic touchscreen. The present invention provides such a method.
The present invention provides a method for distinguishing between contaminants and valid touches in a surface acoustic wave touchscreen. The system compares the measured waveform against a baseline waveform for each axis and validates the new touch if the measured waveform is different from the baseline waveform for either axis. Therefore even if the new touch lies within the shadow created by a contaminant in one axis, the new touch is still validated as it is not within the shadow created by the contaminant in the second axis.
In one embodiment of the invention, after the system determines that the measured response is different from the baseline in at least one axis, the system compares the new touch to one or more selection criteria to insure that it is a valid touch and not due to a contaminant, electrical noise, or other invalid source. Preferably the duration of the new touch is monitored, thus allowing the new touch to be invalidated if the duration is too short, indicating system noise, or too long, indicating, for example, a contaminant. Also preferably the size of the new touch is determined, thus allowing the touch to be invalidated if the size is too large, possibly being indicative of a contaminant or other foreign body resting on the touchscreen surface.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.